Oxygen Sensor Monitoring
Oxygen Sensor Monitoring
The oxygen sensors are monitored for:
- Operational readiness
- Connectivity
- Short circuit to B+
- Short circuit to ECM ground
- Open circuit
- Aging of oxygen sensors via the control frequency and control position
Function of oxygen sensor diagnosis
General
To diagnose the oxygen sensor signals, the DME control module uses the form and frequency of the respective signal as a measuring value.
The data to be calculated for the DME control module are:
- Time of sensor voltage between positive and negative edges
- Maximum recognized sensor voltage value
- Minimum recognized sensor voltage value
- Oxygen sensor value towards rich
- Oxygen sensor value towards lean
- Sensing limits of oxygen sensing system
- Period duration of sensors ahead of TWC
- Sensor voltage ahead of TWC
Function
1. Determination of maximum and minimum sensor voltage
After starting the engine, the values for the previous maximum and minimum are erased. When driving, the absolute minimum and maximum are formed in an engine speed and load range predetermined for diagnosis. These values are recorded by the ECM memory. If the sensor voltage is higher than the presently stored maximum value, it will be overridden by the current sensor voltage. The same applies to the minimum value.
2. Calculation of time between positive and negative edges
If the sensor voltage exceeds the sensing limits, the measurement of the time between the positive and negative edges of the sensor signal is started. If the sensor voltage is lower than the sensing limit, the time counter is stopped. The time between the starting and stopping of the counter is measured.
Monitoring of Sensor Aging
Contamination or aging of the oxygen sensor affects the sensor voltage or period duration of the sensor signal and therefore has a negative influence on emission values.
Illustration of the effect of period duration on emission values.
Monitoring of Oxygen Sensors Ahead of TWC
Sensor readiness
The sensor voltage has to have left the 400 mV to 600 mV range within 200 seconds after the engine is started, and must not be above 1000 mV or below -150 mV.
No relevant interference may be present during this period.
Connectivity (signal wire to signal ground)
When the sensor is cold, the sensor voltage must equal 0 V ± 40 mV. When the sensor has reached operating temperature and the voltage of the sensor after the TWC is greater than 350 mV, the voltage of the sensor ahead of the TWC may lie between 60 mV and 400 mV for no longer than 15 seconds.
Open circuit
After starting the engine and 200 seconds of sensor heating, the sensor voltage may not lie between 400 mV and 600 mV for longer than 5 seconds.
Short circuit to B+
When the heater of the sensor was actuated for more than 200 seconds, the sensor voltage may not exceed 1000 mV.
Short circuit to ground
When the heater of the sensor was actuated for more than 200 seconds, the sensor voltage may not fall below -150 mV. When the sensor is cold, the sensor voltage must not lie in the voltage range from -360 mV to -160 mV.
Monitoring of Oxygen Sensors After TWC
Sensor readiness
The sensor voltage has to have left the 400 mV to 600 mV range within 200 seconds after the engine is started, and must not be above 1000 mV or below -150 mV.
Connectivity (signal wire to signal ground)
When oxygen sensing is activated and the sensor heating was triggered for 200 seconds and if no interference exists such as secondary air, tank ventilation or fault conditions, the sensor voltage must not remain in the range from -40 mV to +60 mV for longer than 200 seconds.
Open circuit
After starting the engine and 200 seconds of sensor heating, the sensor voltage must have passed through the 400 mV to 500 mV range within 400 seconds.
Short circuit to B+
When the heater of the sensor was actuated for more than 200 seconds, the sensor voltage may not exceed 1000 mV.
Short circuit to ground
When the heater of the sensor was actuated for more than 200 seconds, the sensor voltage may not fall below -150 mV. When the sensor is cold, the sensor voltage must not lie in the voltage range from -360 mV to -160 mV.
Oxygen Sensor Heating
A resistor (shunt) is provided in the heating circuit for each oxygen sensor pair ahead of the TWC and for each sensor pair after the TWC. This is located in the DME control module. The monitoring of the heating is accomplished by measuring the sensor heating current (depending on the voltage drop at the shunt) and the voltage for the sensor heating. The DME control module uses these values to calculate the heating resistance.
For right-left recognition, a check is carried out during a cold start to determine whether the sensor readiness of a sensor pair is within a certain time span. If, for example, the left sensor is operationally ready much later than the right sensor, we can conclude that the left sensor has a heating malfunction.